Direct Ink Writing of SiCN/RuO2/TiB2 Composite Ceramic Ink for High-Temperature Thin-Film Sensors

Direct ink writing (DIW) of high-temperature thin-film sensors holds significant potential for monitoring extreme environments. However, existing high-temperature inks face a trade-off between cost and performance. This study proposes a SiCN/RuO2/TiB2 composite ceramic ink. The added TiB2, after annealing in a high-temperature atmospheric environment, forms B2O3 glass, which synergizes with the SiO2 glass phase formed from the SiCN precursor to effectively encapsulate RuO2 particles. This enhances the film’s density and adhesion to the substrate, preventing RuO2 volatilization at high temperatures. Additionally, the high conductivity of TiB2 improves the film’s overall conductivity. Test results indicate that the SiCN/RuO2/TiB2 film exhibits high linearity from room temperature to 900 °C, high stability (resistance drift rate of 0.1%/h at 800 °C), and high conductivity (4410 S/m). As a proof of concept, temperature sensors and a heat flux sensor were successfully fabricated on a metallic hemispherical surface. Performance tests in extreme environments using high-power lasers and flame guns verified that the conformal thin-film sensor can accurately measure spherical temperature and heat flux, with a heat flux sensor response time of 53 ms. In conclusion, the SiCN/RuO2/TiB2 composite ceramic ink developed in this study offers a high-performance and cost-effective solution for high-temperature conformal thin-film sensors in extreme environments.


Introduction
With the rapid development in the aerospace field, the optimization design and simulation require precise in situ surface condition parameters [1,2].However, key components often operate in extreme environments characterized by high temperatures, high pressures, high centrifugal forces, and severe erosion.Traditional discrete sensors are no longer adequate for high-precision measurements in the aerospace industry due to their large size and thermal mismatch issues [3][4][5][6].Thin-/thick-film sensors (TFSs) have emerged as a crucial direction for sensor development in extreme environments because of their small size, high sensitivity, minimal structural damage, and high accuracy [7][8][9][10].Nevertheless, traditional physical vapor deposition (PVD) methods struggle to achieve uniform and effective thin-film deposition on the complex surfaces of aerospace components.In recent years, direct ink writing technology has emerged as a promising method for conformal deposition of high-temperature thin-film sensors on complex curved surfaces.This method represents a significant advancement in addressing the challenges of thin-film deposition in aerospace applications [11][12][13].
Currently, the ink materials commonly used for high-temperature sensors are predominantly precious metals (such as AgPd, Pt, PtRh) [14][15][16][17], but the high cost of these materials restricts their widespread application.Although ceramic inks [12,18,19] (such as TiB 2 , indium tin oxide (ITO) [20,21], etc.) have been developed, thin-film sensors made from Materials 2024, 17, 3792 2 of 12 these ceramics exhibit poor linearity, limiting their engineering applications.RuO 2 , known for its excellent electrical conductivity, high-temperature resistance, and high linearity, has been widely utilized in integrated circuits and electronic components.However, RuO 2 tends to oxidize to gaseous RuO 4 at high temperatures [22], leading to instability in the electrical performance of the sensors.Current encapsulation materials such as silicate glass or SiO 2 can enhance the stability of RuO 2 , but these materials often reduce their conductivity [23][24][25].Hence, there is an urgent need to develop a composite ink that can enhance the high-temperature stability of RuO 2 without compromising its conductivity.Ceramic precursor polymers can be converted into conductive amorphous ceramics upon high-temperature pyrolysis [26][27][28][29], making them an excellent choice for ink carriers.However, ceramic precursor polymers undergo up to 60% shrinkage during high-temperature pyrolysis [30,31], and thin films made solely from RuO 2 and ceramic precursors are prone to cracking and detachment.Adding fillers is an effective strategy to prevent thin-film cracking and enhance film adhesion [32].TiB 2 , as a non-oxide ceramic, generates TiO 2 and B 2 O 3 at high temperatures [11], which not only reduces film cracking but also improves the conductivity of the film, making it an optimal material for preparing RuO 2 composite inks.
In this study, we developed a SiCN precursor and RuO 2 /TiB 2 composite ink, successfully applying it to the fabrication of thin-film heat flux sensors and resistance thermometers on metallic hemispherical surfaces.This composite ink utilizes the SiCN precursor as the carrier and RuO 2 as the primary conductive component.The synergistic effect of B 2 O 3 generated from TiB 2 and the SiO 2 glass phase converted from the SiCN precursor enhances film density, encapsulates RuO 2 to reduce its volatilization, and adjusts the thermal matching of the film layer, thereby improving its adhesion to the substrate.Meanwhile, TiB 2 also enhances the film's conductivity.High-temperature tests demonstrated that the SiCN/RuO 2 /TiB 2 film exhibited excellent high-temperature resistance, with high linearity of resistance from room temperature to 900 • C, good stability (0.1%/h resistance drift at 800 • C), and high conductivity (4410 S/m).As a proof of concept, we printed SiCN/RuO 2 / TiB 2 ink on a metallic hemispherical surface to create conformal heat flux sensors and resistance thermometers.Extreme environment tests using high-power lasers and butane torches confirmed that the SiCN/RuO 2 /TiB 2 composite thin-film sensors could accurately monitor the surface temperature and heat flux of metal components.

Materials
The RuO 2 powder, with an average particle size of 200 nm, was sourced from Suzhou Sub-Nano New Material Co., Ltd., Suzhou, China.High-temperature resistant Ag ink was procured from Chengdu Hehe Micro-Nano Technology Co., Ltd.(Chengdu, China).Titanium diboride (TiB 2 ) powder, featuring a particle size of 1 µm, was obtained from Shanghai Chaowei Nanomaterials Co. PSN2 (Shanghai, China) [33], the precursor of SiCN ceramics, was commercially acquired from the Institute of Chemistry, Chinese Academy of Sciences (Beijing, China).

Fabrication Method
The SiCN/RuO 2 /TiB 2 composite ink was formulated by thoroughly mixing RuO 2 micron-scale powder and TiB 2 micron-scale powder with PSN2 as the organic solvent in a 2:1:1 ratio, utilizing a mixer for one hour (Figure 1a).The SiCN/RuO 2 /TiB 2 composite ink was loaded onto a Wittenburg technology-based 3D direct writing platform [34] equipped with a 330 µm nozzle.The direct writing platform was used to print the SiCN/RuO 2 /TiB 2 composite ink on a curved surface.The motor speed of the platform was set to 3000 rpm, with a movement speed of 0.2 mm/s.The microneedle used had a diameter of 160 µm, and the distance between the needle tip and the curved surface was maintained at 5 µm.The pattern to be printed was designed using CAD software(Version T.53.0.0 AutoCAD 2023).

Characterization Methods
Scanning electron microscope (SEM) images were captured using a Zeiss S (Oberkochen, Germany).X-ray diffraction (XRD) analysis was conducted us madzu XRD-6100 instrument (Kyoto, Japan).Thermoelectric potential mea were performed using a Keysight 34972A data acquisition system (DAQ)(Santa USA) within a tube furnace.Temperature was recorded with a commercially av type thermocouple (Taizhou China).An infrared thermal imager, Fluke Ti480 P ett, WA, USA), capable of measuring temperatures up to 1000 °C, was employe mal imaging.

Testing Methods
The temperature-resistance test system comprised a quartz tube furnace (O MTI KJ GROUP, Hefei China), a data acquisition device (KEYSIGHT 34972A) type thermocouple (KPS-K, Taizhou China).The K-type thermocou SiCN/RuO2/TiB2 composite resistor were placed together in the constant temper of the tube furnace, with signals from both the thermocouple and resistor being simultaneously.The heat flux test system included a data acquisition unit (K 34972A) and a 1064 nm laser, which was directly aimed at the sensor surface power was controlled to adjust the heat flux input.

Sintering and Characteristics of the Conformal High-Temperature Sensitive Layer
High-temperature sintering is crucial for the performance of the SiCN/ composite ink under elevated temperatures.Figure 2a analyzes the mor changes of the composite film after sintering at different temperatures.After 300 °C for 1 h, no significant sintering was observed, and the film thickness, me a profilometer, was approximately 49 µm.TiB2 began to oxidize to TiO2.At arou TiB2 oxidation produced B2O3 [11].The XRD images did not show the presen because it is amorphous.After sintering at 800 °C for 1 h, significant changes served on the film surface.The SiCN underwent a glass transition, forming SiO sulating properties at high temperatures.Together with the TiO2 and B2O3 prod the oxidation of TiB2, these oxides encapsulated the RuO2 particles, preventing dation to volatile RuO4.At this stage, the film shrank longitudinally by approxim (Figure 2b), resulting in a more compact structure and the formation of c

Characterization Methods
Scanning electron microscope (SEM) images were captured using a Zeiss Sigma 300.(Oberkochen, Germany).X-ray diffraction (XRD) analysis was conducted using a Shimadzu XRD-6100 instrument (Kyoto, Japan).Thermoelectric potential measurements were performed using a Keysight 34972A data acquisition system (DAQ)(Santa Rosa, CA, USA) within a tube furnace.Temperature was recorded with a commercially available K-type thermocouple (Taizhou China).An infrared thermal imager, Fluke Ti480 PRO (Everett, WA, USA), capable of measuring temperatures up to 1000 • C, was employed for thermal imaging.

Testing Methods
The temperature-resistance test system comprised a quartz tube furnace (OTF-1200X, MTI KJ GROUP, Hefei China), a data acquisition device (KEYSIGHT 34972A), and a K-type thermocouple (KPS-K, Taizhou China).The K-type thermocouple and SiCN/RuO 2 /TiB 2 composite resistor were placed together in the constant temperature zone of the tube furnace, with signals from both the thermocouple and resistor being collected simultaneously.The heat flux test system included a data acquisition unit (KEYSIGHT 34972A) and a 1064 nm laser, which was directly aimed at the sensor surface.The laser power was controlled to adjust the heat flux input.

Sintering and Characteristics of the Conformal High-Temperature Sensitive Layer
High-temperature sintering is crucial for the performance of the SiCN/RuO 2 /TiB 2 composite ink under elevated temperatures.Figure 2a analyzes the morphological changes of the composite film after sintering at different temperatures.After holding at 300 • C for 1 h, no significant sintering was observed, and the film thickness, measured by a profilometer, was approximately 49 µm.TiB 2 began to oxidize to TiO 2 .At around 400 • C, TiB 2 oxidation produced B 2 O 3 [11].The XRD images did not show the presence of B 2 O 3 because it is amorphous.After sintering at 800 • C for 1 h, significant changes were observed on the film surface.The SiCN underwent a glass transition, forming SiO 2 with insulating properties at high temperatures.Together with the TiO 2 and B 2 O 3 produced from the oxidation of TiB 2 , these oxides encapsulated the RuO 2 particles, preventing their oxidation to volatile RuO 4 .At this stage, the film shrank longitudinally by approximately 50% (Figure 2b), resulting in a more compact structure and the formation of conductive pathways within the film.The addition of TiB2 increased the film's conductivity to 4410 S/m (Figure 2c), which is higher than previously reported values [23].We also tested the performance of the SiCN/RuO 2 composite ink.We found that the conductivity of the film sintered from the SiCN/RuO 2 composite ink was lower compared to the SiCN/RuO 2 /TiB 2 composite film (Figures S1 and S2).Additionally, the SiCN/RuO 2 film exhibited higher porosity and lower density.After sintering at 900 • C, more cracks appeared on the composite film surface, and RuO 2 particles began to be exposed.The film thickness further decreased, likely due to the volatilization of some SiO 2 -B 2 O 3 -TiO 2 ternary oxides at high temperatures, allowing some oxygen to diffuse into the film at high temperatures [35], leading to a reduction in conductivity to approximately 1555 S/m.After sintering at 1000 • C for 1 h, the SiO 2 , B 2 O 3 , and other oxides continued to volatilize, exposing more RuO 2 particles on the film surface.Some RuO 2 particles volatilized at high temperatures, reducing the film thickness to only 7.5 µm.The conductivity decreased to 219.8 S/m, and XRD analysis indicated the formation of significant amounts of Ti 0.1 Ru 0.9 O 2 in the sample sintered at 1000 • C.
pathways within the film.The addition of TiB2 increased the film's conductivity to 4410 S/m (Figure 2c), which is higher than previously reported values [23].We also tested the performance of the SiCN/RuO2 composite ink.We found that the conductivity of the film sintered from the SiCN/RuO2 composite ink was lower compared to the SiCN/RuO2/TiB2 composite film (Figures S1 and S2).Additionally, the SiCN/RuO2 film exhibited higher porosity and lower density.After sintering at 900 °C, more cracks appeared on the composite film surface, and RuO2 particles began to be exposed.The film thickness further decreased, likely due to the volatilization of some SiO2-B2O3-TiO2 ternary oxides at high temperatures, allowing some oxygen to diffuse into the film at high temperatures [35], leading to a reduction in conductivity to approximately 1555 S/m.After sintering at 1000 °C for 1 h, the SiO2, B2O3, and other oxides continued to volatilize, exposing more RuO2 particles on the film surface.Some RuO2 particles volatilized at high temperatures, reducing the film thickness to only 7.5 µm.The conductivity decreased to 219.8 S/m, and XRD analysis indicated the formation of significant amounts of Ti0.1Ru0.9O2 in the sample sintered at 1000 °C.The EDS results (Figure 3) corroborated these observations.After sintering the composite ink at 800 °C for 1 h, Si, Ru, and O were evenly distributed on the substrate surface, with O being the most abundant, while Ti and Si were present in the lowest proportions.By EDS analysis, it can be seen that the distribution of Ti elements is non-continuous, while Ru elements are uniformly and continuously distributed in the film.This indirectly indicates that the conductive pathways are formed by the interconnected RuO2 particles, and the randomly distributed TiB2, due to its good conductivity, plays a role in reducing The EDS results (Figure 3) corroborated these observations.After sintering the composite ink at 800 • C for 1 h, Si, Ru, and O were evenly distributed on the substrate surface, with O being the most abundant, while Ti and Si were present in the lowest proportions.By EDS analysis, it can be seen that the distribution of Ti elements is non-continuous, while Ru elements are uniformly and continuously distributed in the film.This indirectly indicates that the conductive pathways are formed by the interconnected RuO 2 particles, and the randomly distributed TiB 2 , due to its good conductivity, plays a role in reducing electron mobility resistance.This is equivalent to increasing the cross-sectional area of the conductive paths, thus enhancing conductivity.The distribution characteristic of O elements suggests that SiO 2 , generated during the ceramicization of SiCN at high temperatures, coordinated with B 2 O 3 and TiO 2 generated by TiB 2 to encapsulate the RuO 2 particles, thereby preventing their volatilization in high-temperature environments.Consistent with the planar results, the cross-sectional EDS analysis revealed that Ru and O had the highest content, while Ti was the least abundant.The tendency of Ti particles to increase in size at high temperatures was evident.The boundary between the composite film and the alumina substrate in the EDS analysis was visible, exhibiting a continuous interface without pores, indicating excellent film quality.The composite film demonstrated good thermal expansion matching performance.These results indicate that the ternary thermally grown oxide of SiO 2 -B 2 O 3 -TiO 2 , generated from the SiCN material and TiB 2 at high temperatures, enhances the overall performance of the SiCN/RuO 2 /TiB 2 composite film.As clearly shown in Figure 3c, during the sintering process of the SiCN/RuO 2 /TiB 2 composite ink at 800 • C, PSN2 decomposes at high temperatures to form SiO 2 .This SiO 2 , along with B2O 3 and TiO 2 produced from the partial oxidation of TiB 2 , encapsulates the RuO2 particles, preventing O 2 from penetrating the interior of the film.Concurrently, the film becomes thinner and denser, which facilitates the formation of conductive pathways between RuO 2 and TiB 2 , thereby enhancing the electrical conductivity.
electron mobility resistance.This is equivalent to increasing the cross-sectional area of the conductive paths, thus enhancing conductivity.The distribution characteristic of O elements suggests that SiO2, generated during the ceramicization of SiCN at high temperatures, coordinated with B2O3 and TiO2 generated by TiB2 to encapsulate the RuO2 particles, thereby preventing their volatilization in high-temperature environments.Consistent with the planar results, the cross-sectional EDS analysis revealed that Ru and O had the highest content, while Ti was the least abundant.The tendency of Ti particles to increase in size at high temperatures was evident.The boundary between the composite film and the alumina substrate in the EDS analysis was visible, exhibiting a continuous interface without pores, indicating excellent film quality.The composite film demonstrated good thermal expansion matching performance.These results indicate that the ternary thermally grown oxide of SiO2-B2O3-TiO2, generated from the SiCN material and TiB2 at high temperatures, enhances the overall performance of the SiCN/RuO2/TiB2 composite film.As clearly shown in Figure 3c, during the sintering process of the SiCN/RuO2/TiB2 composite ink at 800 °C, PSN2 decomposes at high temperatures to form SiO2.This SiO2, along with B2O3 and TiO2 produced from the partial oxidation of TiB2, encapsulates the RuO2 particles, preventing O2 from penetrating the interior of the film.Concurrently, the film becomes thinner and denser, which facilitates the formation of conductive pathways between RuO2 and TiB2, thereby enhancing the electrical conductivity.

Testing the Thermistor Performance of SiCN/RuO 2 /TiB 2 Composite Ink Film
The ceramicization of the SiCN precursor at high temperatures, along with the production of B 2 O 3 and TiO 2 from TiB 2 , collectively contributes to filling the gaps among RuO 2 particles.This increases the film density, forms conductive paths, and ensures stable conductivity at high temperatures.Figure 4a depicts samples fabricated by 3D printing technology, where SiCN/RuO 2 /TiB 2 composite ink is deposited onto aluminum oxide plates for temperature testing purposes.Testing the composite ink film sensor over four thermal cycles from room temperature (30 • C) to 900 • C revealed that the thermistor curve between 30 • C and 900 • C demonstrated a high fit with an R 2 value of 0.999 (Figure 4c), indicating excellent high-temperature resistance for the composite film.Few materials [16,[36][37][38][39] have been able to maintain such a good linear relationship over such a wide temperature range, making this composite a potential substitute for precious metals in high-temperature sensor applications.
The ceramicization of the SiCN precursor at high temperatures, along with the production of B2O3 and TiO2 from TiB2, collectively contributes to filling the gaps among RuO2 particles.This increases the film density, forms conductive paths, and ensures stable conductivity at high temperatures.Figure 4a depicts samples fabricated by 3D printing technology, where SiCN/RuO2/TiB2 composite ink is deposited onto aluminum oxide plates for temperature testing purposes.Testing the composite ink film sensor over four thermal cycles from room temperature (30 °C) to 900 °C revealed that the thermistor curve between 30 °C and 900 °C demonstrated a high fit with an R 2 value of 0.999 (Figure 4c), indicating excellent high-temperature resistance for the composite film.Few materials [16,[36][37][38][39] have been able to maintain such a good linear relationship over such a wide temperature range, making this composite a potential substitute for precious metals in high-temperature sensor applications.
To verify its durability at high temperatures, a resistivity change curve at 800 °C for over 30 h was measured (Figure 4d), showing a resistance drift rate of only 0.1%/h at 800 °C.This confirms the exceptional high-temperature stability of the composite film, surpassing the stability of previously reported materials.The output characteristics of the film's thermistor under alternating temperatures between 700 °C and 900 °C for over 10 h were also measured.The output results were nearly identical to the temperature signal reflected by the type-K thermocouple, confirming the composite material's excellent stability at high temperatures (Figure 4e).

Application Verification of the SiCN/RuO2/TiB2 Composite Ink Conformal Thin-Film Thermistor Array
To further illustrate the composite material's distributed temperature monitoring characteristics on complex surfaces under extreme conditions, lasers and flame guns were employed to simulate such environments.A 1064 nm fiber continuous laser was used to To verify its durability at high temperatures, a resistivity change curve at 800 • C for over 30 h was measured (Figure 4d), showing a resistance drift rate of only 0.1%/h at 800 • C.This confirms the exceptional high-temperature stability of the composite film, surpassing the stability of previously reported materials.The output characteristics of the film's thermistor under alternating temperatures between 700 • C and 900 • C for over 10 h were also measured.The output results were nearly identical to the temperature signal reflected by the type-K thermocouple, confirming the composite material's excellent stability at high temperatures (Figure 4e).

Application Verification of the SiCN/RuO 2 /TiB 2 Composite Ink Conformal Thin-Film Thermistor Array
To further illustrate the composite material's distributed temperature monitoring characteristics on complex surfaces under extreme conditions, lasers and flame guns were employed to simulate such environments.A 1064 nm fiber continuous laser was used to directly irradiate a thermistor located in the upper left corner of the thermistor array, with the power incrementally increased by 15 W every 60 s, up to a maximum of 105 W (Figure 5a).Simultaneously, the infrared thermal imager records the temperature data at the 60th second after each power change (Figure 5d).The resistance of four thermistors was recorded in real time using four conformal Ag film wires, and the fitting formula from Figure 4c was used to calculate the temperature of the measured sensitive layer.The temperature calculated from the thermistor directly irradiated by the laser was 757.75 • C, closely matching the 747.

Application Test of the Conformal Film Heat Flux Sensor
Measuring temperature alone cannot adequately reflect the dynamic process of heating or cooling of components under extreme conditions.Heat flux measurement makes To demonstrate the temperature difference caused by a high-temperature gas flow impact between using a discrete thermocouple and a conformal thermistor array, a Ktype thermocouple was affixed to the sensitive layer.A butane flame torch was aimed at the middle position of the spherical sensitive layer while simultaneously collecting data from the thermistor and the K-type thermocouple.As shown in Figure 5e, the K-type thermocouple reading instantly rose to a high temperature of 980 • C, while the maximum temperature calculated from the thermistor was only 319 • C. The K-type thermocouple has a low thermal mass itself, so it instantly rises above 900 • C when affected by the flame impact.However, the metallic hemisphere has a larger thermal mass, causing it to heat up more slowly.The surface temperature was only about 319 • C, while the infrared thermal imager displayed a metallic hemisphere surface temperature of approximately 317.7 • C. The temperature measurement results of the conformal film sensor are closer to the actual temperature of the surface of the structural component.
Traditional discrete temperature sensors often require modification of the original structure for installation, disrupting the consistency and continuity of heat conduction in the component.This results in severe " cold/hot spot effects " during the measurement process, causing the measured temperature to often not reflect the true temperature of the component.However, a conformal film temperature sensor can avoid this problem due to its micrometer-level thickness and in situ manufacturing method, thereby achieving precise in situ temperature monitoring.
Experimental proofs show that under the two extreme conditions of laser and flame gun exposure, the SiCN/RuO 2 /TiB 2 composite ink conformal thin-film heat-sensitive resistor exhibits excellent high-temperature stability.This verifies that the composite material has the potential to serve as a new raw material for high-temperature sensors.

Application Test of the Conformal Film Heat Flux Sensor
Measuring temperature alone cannot adequately reflect the dynamic process of heating or cooling of components under extreme conditions.Heat flux measurement makes the heat transfer process more evident and accurate [40,41].The sensitive layer pattern designed in this work, based on the Wheatstone bridge principle, can function as a SiCN/RuO 2 /TiB 2 composite ink temperature sensing array or as a conformal film heat flux sensor.Figure 6b illustrates the structural characteristics of the heat flux sensor, which comprises the metal substrate, electrical insulation coating, sensitive layer, packaging layer, and thermal resistance layer.When the heat flux sensor receives heat flow, the different resistances in the sensitive layer's four resistors, caused by the thermal resistance layer's thickness, create a differential output.
The sensitive layer is connected to a direct current voltage, converting resistance changes into output voltage changes in the circuit.The heat flow signal is provided by a 1064 nm fiber laser (Figure 6a).To improve the conformal sensor's surface absorption rate, the surface is typically made black, as the encapsulated layer and thermal resistance layer are made of transparent glaze, affecting the laser absorption rate.This setup yields experimental data, as shown in Figure 6c,d, where gradually increasing the laser power results in a progressively increasing output voltage signal.A continuous heat flux measurement over 21 cycles with 5 s intervals demonstrated the sensor's excellent high-temperature durability and stability.The response time of the conformal heat flux sensor, made of SiCN/RuO 2 /TiB 2 composite material, is remarkably fast at only 53 ms, verifying the sensor's broad application prospects.Furthermore, a rapid impact was applied to the center of the metallic hemispherical surface using a butane flame torch (Figure 6f), resulting in a significant heat flow signal (Figure 6g).This testing reaffirms the potential of the SiCN/RuO 2 /TiB 2 composite ink as a robust material for high-temperature sensors.
SiCN/RuO2/TiB2 composite material, is remarkably fast at only 53 ms, verifying the sensor's broad application prospects.Furthermore, a rapid impact was applied to the center of the metallic hemispherical surface using a butane flame torch (Figure 6f), resulting in a significant heat flow signal (Figure 6g).This testing reaffirms the potential of the SiCN/RuO2/TiB2 composite ink as a robust material for high-temperature sensors.

Conclusions
In summary, this study addresses the volatility and instability of RuO 2 at high temperatures by proposing a SiCN/RuO 2 /TiB 2 composite ink with stable performance in high-temperature environments.The composite ink achieves a denser and more adherent high-quality film by filling the pores between RuO 2 particles with SiO 2 -B 2 O 3 -TiO 2 ternary thermally grown oxides generated from the SiCN precursor and TiB 2 at high temperatures.These glass phases also reduce the sintering temperature of RuO 2 , thereby minimizing its volatility under high-temperature conditions.Additionally, the incorporation of TiB 2 enhances the film's conductivity (4410 S/m) and prevents film delamination and cracking.Furthermore, thermal sensitivity tests on the SiCN/RuO 2 /TiB 2 composite film thermistors in high-temperature environments reveal that the resistance-temperature fitting curve exhibits a high correlation (R 2 = 0.999) from room temperature to 900 • C. The film can operate stably for over 30 h at 800 • C, with a resistance drift rate of only 0.1% per hour, demonstrating the composite film's potential for engineering applications.To verify the technical feasibility of applying this composite film to conformal film sensors, high-power laser and butane flame torch tests were conducted on a conformal sensor array on the metallic hemispherical surface for extreme environment evaluations of thermistor and heat flux performance.The results indicate that the thermistor array can accurately measure the temperature at different points on the metallic hemispherical surface, showing significant advantages over discrete sensors.The heat flux sensor exhibits a response time of only 53 ms and demonstrates excellent high-temperature durability and stability.Through

Figure 2 .
Figure 2. Surface characteristics of the SiCN/RuO2/TiB2 composite film.(a) Surface morphology of the composite film at different sintering temperatures.(b) Cross-sectional changes of the film after annealing at different sintering temperatures.(c) Graph of conductivity versus sintering temperature.(d) XRD pattern.

Figure 2 .
Figure 2. Surface characteristics of the SiCN/RuO 2 /TiB 2 composite film.(a) Surface morphology of the composite film at different sintering temperatures.(b) Cross-sectional changes of the film after annealing at different sintering temperatures.(c) Graph of conductivity versus sintering temperature.(d) XRD pattern.

Figure 3 .
Figure 3. (a)Surface morphology and EDS elemental mapping of the printed SiCN/RuO 2 /TiB 2 composite film.(b) Cross-sectional surface morphology and EDS elemental mapping of the film sintered at 800 • C for 1 h.(c) Sintering process schematic diagram.

Figure 4 .
Figure 4. Thermistor performance testing of the SiCN/RuO2/TiB2 composite film thermistor.(a) Test object image.(b) Cold and hot cycle test.(c) Second-order fitting of experimental data.(d) Test results after 30 h of holding at 800 °C.(e) Durability test under high-temperature cyclic conditions.

Figure 4 .
Figure 4. Thermistor performance testing of the SiCN/RuO 2 /TiB 2 composite film thermistor.(a) Test object image.(b) Cold and hot cycle test.(c) Second-order fitting of experimental data.(d) Test results after 30 h of holding at 800 • C. (e) Durability test under high-temperature cyclic conditions.
2 • C displayed by the infrared thermal imager.The other three thermistors displayed temperatures of 582.14 • C, 362.29 • C, and 287.87 • C, respectively, with temperature differences influenced by the distance between the thermistor and the laser's direct spot.Materials 2024, 17, x FOR PEER REVIEW 8 of 12

Figure 5 .
Figure 5. Harsh environment testing of the SiCN/RuO2/TiB2 composite material thermistor array.(a) Schematic of laser irradiation on the metallic hemispherical surface thermistor array.(b) Thermistor array design scheme.(c) Test results of gradually increasing laser power on a single thermistor on the metallic hemispherical surface.(d) Infrared thermal imaging at 60s under a certain power of laser irradiation.(e) Comparison of conformal film thermistor and K-type thermocouple under the flame gun impact.(f) Infrared thermal imaging under flame gun impact.

Figure 5 .
Figure 5. Harsh environment testing of the SiCN/RuO 2 /TiB 2 composite material thermistor array.(a) Schematic of laser irradiation on the metallic hemispherical surface thermistor array.(b) Thermistor array design scheme.(c) Test results of gradually increasing laser power on a single thermistor on the metallic hemispherical surface.(d) Infrared thermal imaging at 60s under a certain power of laser irradiation.(e) Comparison of conformal film thermistor and K-type thermocouple under the flame gun impact.(f) Infrared thermal imaging under flame gun impact.

Figure 6 .
Figure 6.(a) Schematic of laser irradiation on the metallic hemispherical surface heat flux sensor.(b) Structural principle schematic of the heat flux sensor.(c) Test results under different heat fluxes.(d) Test results of 21 cycles under the same heat flux.(e) Response time test results.(f) Optical image during flame gun impact test.(g) Test results of the heat flux sensor under flame gun impact.

Figure 6 .
Figure 6.(a) Schematic of laser irradiation on the metallic hemispherical surface heat flux sensor.(b) Structural principle schematic of the heat flux sensor.(c) Test results under different heat fluxes.(d) Test results of 21 cycles under the same heat flux.(e) Response time test results.(f) Optical image during flame gun impact test.(g) Test results of the heat flux sensor under flame gun impact.